The physical theory of lubrication is now well understood; see, for example:
Bondi, A., Physical Chemistry of Lubricating Oils, Reinhold Publishing Corp., New York, N.Y. (1951); PA0 Cameron, A., The Principles of Lubrication, Wiley & Sons, Inc., New York, N.Y. (1967); PA0 Ferry, John D., Viscoelastic Properties of Polymers, Wiley & Sons, Inc., New York, N.Y. (1961).
As described in these and other standard works on the subject, three distinct types of lubrication occur, namely hydrodynamic, thin-film and boundary lubrication. Hydrodynamic lubrication occurs when there is a continuous thick film of fluid totally separating the surfaces moving relative to one another, this film being much thicker than any surface irregularities or roughness on the moving surfaces. The coefficient of friction is very low, typically of the order of 0.001, and no wear occurs. This is, of course, the ideal lubrication condition and if it could be maintained the moving parts would last indefinitely.
As the load applied to the relatively moving parts is increased, the thickness of the lubricant film decreases, so that thin-film or elastohydrodynamic lubrication appears. In this type of lubrication, the film is typically of the order of two microns thick and the hydrodynamic pressure exerted by the film is increased and is sufficient to deform the adjacent substrates. This increased localized pressure has the profound and useful effect of increasing the viscosity of the lubricant. As shown in standard works on the subject, for example:
Partington, J. R., An Advanced Treatise on Physical Chemistry, Volume II, Longmans (1962), the increase in lubricant viscosity with pressure is governed by the Warburg-Sachs equation: EQU n.sub.2 =n.sub.1 [1-a(P.sub.2 -P.sub.1)]
where n.sub.1 and n.sub.2 are the viscosities at pressures P.sub.1 and P.sub.2 respectively and a is a constant for a given fluid and is known as the viscosity/pressure coefficient.
In thin-film lubrication, almost all the physical properties of the fluid play a role i.e. viscosity/pressure coefficient, viscosity/temperature coefficient, thermal conductivity etc. are all important in determining the behavior of a lubricant in thin-film lubrication.
The third type of lubrication, boundary lubrication, occurs under severe conditions of high load per unit area and low relative speed between the relatively moving surfaces. Under these severe conditions, it is impossible to maintain adequate lubrication and the lubricant is essentially absorbed onto the surfaces to form protective films. Because the lubricant action is that of the absorbed films, the bulk properties of the lubricant fluid, such as viscosity, are relatively unimportant. Instead, the chemical interaction between the lubricant and the surface is the critical parameter. Accordingly, when it is essential to rely upon this type of lubrication, extreme pressure additives are added to the fluid. Such additives generally contain a moiety, for example chlorine or phosphorus, which reacts with the metals to provide a protective, inorganic lubricating surface layer.
In many commercially important applications, the loadings on the moving parts are such that it is not practical to maintain hydrodynamic lubrication, but it is practical to maintain thin-film lubrication; for obvious reasons, boundary lubrication is avoided unless it is absolutely necessary. Accordingly, the viscosity/pressure coefficient is an important parameter of lubricant compositions used in such applications and a high value of this coefficient is desirable in order that the viscosity will increase rapidly with pressure and provide good protection to the lubricated surfaces.
Mineral oils are, of course, known to be good lubricants and typically have viscosity/pressure coefficients of about 4.8.times.10.sup.-4 atm..sup.-1 at 30.degree. C., as noted in Partington, supra. Water is an extremely bad lubricant and actually has a negative viscosity pressure coefficient of -1.7.times.10.sup.-4 atm..sup.-1 at 25.degree. C., as noted in Partington, supra.
Pure ethylene glycol is a good lubricant having a viscosity/pressure coefficient of 4.3.times.10.sup.-4 atm..sup.-1 at 25.degree. C. Diethylene glycol has an even better viscosity/pressure coefficient of 4.6.times.10.sup.-4 atm..sup.-1 at the same temperature. Unfortunately, the addition of even modest amounts of water to ethylene or diethylene glycol drastically reduces the viscosity/pressure coefficient of the glycol; mixtures of 44 weight percent ethylene or diethylene glycol with 56 weight percent water have a viscosity/pressure coefficient of zero. Since it is in practice impossible to keep ethylene or diethylene glycol anhydrous under industrial conditions (the pure materials are hygroscopic), the dramatic drop in viscosity/pressure coefficient on addition of water essentially destroys the potential usefulness of pure ethylene and diethylene glycols as lubricants.
It is known that the rapid drop in viscosity/pressure coefficient occurring when ethylene or diethylene glycol is diluted with water can be retarded by adding to the glycol/water mixture a substantial proportion of a polyalkylene glycol. One such glycol used commercially in water-based lubricating compositions is sold by Union Carbide Corporation under the Registered Trademark UCON Fluid 75-H-90M. This material is essentially a linear copolymer of 75 percent ethylene oxide and 25 percent propylene oxide having an average molecular weight of approximately 10300 and a neat viscosity of approximately 90000 Saybolt universal seconds (sus.) at 37.8.degree. C. The only side chains on the essentially linear copolymer are due to the methyl groups of the propylene oxide residues. A lubricant composition containing 20 weight percent of this linear polymer, 35 weight percent ethylene glycol and 45 weight percent water has a viscosity pressure coefficient of 3.09.times.10.sup.-4 atm..sup.-1 at 25.degree. C., while the corresponding composition using diethylene glycol has a viscosity pressure coefficient of 3.39.times.10.sup.-4 atm..sup.-1 at the same temperature.
For economic and safety reasons, there is a great demand to increase the proportion of water in water-based lubricant compositions; the 20 percent of the linear polymer used in the compositions just discussed comprises by far the greater proportion of the cost of lubricant composition. Unfortunately, the viscosity of the linear polymer/ethylene (or diethylene) glycol/water lubricant compositions falls very rapidly as the proportion of water is increased, so that as a practical matter the proportion of water cannot be increased above about 50 percent. In contrast, increasing the proportion of water decreases the flammability of the lubricant composition.
In experiments leading to the present invention, numerous modifications of the linear polymer/glycol/water lubricants discussed above were made in an attempt to produce a lubricant containing a higher proportion of water. It was found that the viscosity/pressure coefficient was only slightly affected by the type of glycol, amount of glycol and molecular weight of the polymer. Various changes in the linear polymer, for example in the starter alcohol used, (ethylene oxide:propylene oxide) ratio and the oxide sequencing, also only had marginal effects on the viscosity/pressure coefficient. Accordingly, it appeared that the viscosity/pressure coefficient was primarily dependent upon the water concentration and that achieving high water content with this type of lubricant composition was highly unlikely.
It would also be desirable, if possible, to reduce or eliminate the relatively expensive glycol used in such lubricant compositions, if this could be done without greatly increasing the polymer content of the composition, and polymers intended for use in such glycol-free aqueous lubricant compositions have recently been marketed commercially.
There is thus a need for a water-based lubricant composition which will perform satisfactorily at higher water contents than prior art water-based lubricant compositions, and this invention provides such a lubricant composition and a method for its use.